The present invention relates to intermetallic materials based on MoSi.sub.2, intermetallic matrix composites and methods of making the same. More particularly, the invention is directed to a MoSi.sub.2 based material having an engineered micro-structure provided through the use of in-situ reinforcement whiskers.
MoSi.sub.2 is an attractive intermetallic for structural applications due to its excellent high-temperature oxidation resistance, low density and high thermal conductivity. However, it is brittle at low temperatures, weak at high temperatures and suffers from accelerated oxidation at intermediate temperatures. The accelerated oxidation of MoSi.sub.2 at intermediate temperatures causes the material to disintegrate into powder, a phenomenon known as pesting.
Pesting is a general term describing the catastrophic oxidation of intermetallic materials at intermediate temperatures. The accelerated oxidation leads to the disintegration of the material and component failure. For MoSi.sub.2 the temperature at which pesting is most pronounced is approximately 500.degree. C. It has been observed that at 500.degree. C. bulk (i.e., non-composite) MoSi.sub.2, as well as, composites of MoSi.sub.2 with alumina and aluminum nitride also suffer total disintegration within relatively short time periods, e.g. 100 hours.
The pested samples yield powdery products consisting of MoO.sub.3 whiskers, SiO.sub.2 clusters, and residual MoSi.sub.2. The MoO.sub.3 whiskers exhibited protruding characteristics and were concentrated at the grain boundaries and cracks. The pesting phenomenon in MoSi.sub.2 has been concluded to be the result of the formation of voluminous molybdenum oxides in microcracks. While not wanting to be bound by theory, the accelerated oxidation apparently involves the simultaneous formation of MoO.sub.3 and SiO.sub.2 in amounts essentially determined by the Mo and Si concentrations in the intermetallic.
The addition of about 30 to 50 volume percent of Si.sub.3 N.sub.4 particulate to MoSi.sub.2 reduced the pesting by forming a protective oxide scale as disclosed in assignee's related U.S. Pat. No. 5,429,997, the teachings of which are hereby incorporated by reference. In addition, improvements in room temperature fracture toughness, reductions in the 1200.degree. C. compressive creep rates and lowered coefficient of thermal expansion were attained. Additional improvements in toughness and elevated temperature strength were achieved by reinforcing the MoSi.sub.2 --Si.sub.3 N.sub.4 matrix with about 30 volume percent of silicon carbide continuous fibers. The use of fiber reinforcement is not entirely satisfactory due to the high costs of the present state-of-the-art techniques for making fiber reinforced, composites.
A further difficulty with the use of fiber reinforcement is the coefficient of thermal expansion mismatch between MoSi.sub.2 and most potential reinforcing materials. MoSi.sub.2 has a relatively high coefficient of thermal expansion as compared to most potential reinforcing materials such as silicon carbide fibers. The coefficient of thermal expansion mismatch between the fiber and the matrix material tends to result in matrix cracking during fabrication and severe matrix cracking during thermal cycling which in turn results in component failure. Possible reinforcing fibers include high strength ceramic fibers such as silicon carbide, single crystal alumina, and ductile, high strength molybdenum and tungsten alloy fibers. Ductile niobium fibers have shown improvements in low temperatures strength and toughness, but a severe reaction between the fiber and MoSi.sub.2 limits its use and, in any case, it does not provide improved high-temperature characteristics. The addition of silicon carbide whiskers has yielded improvements in room temperature toughness, but pesting and coefficient of thermal expansion mismatch continue to be problems.